Genes, Exposure to Toxins Linked as Causes of AD/HD

It's well known that exposure to lead and other toxic substances can impair a child's brain function, but children who carry certain genes face greater harm from these environmental toxins. That's the word from researchers at Cincinnati Children's Hospital Medical Center, whose new study suggests that ADHD is caused by a complex interaction of genetic and environmental factors.

"This study shows that certain groups of children have an increased sensitivity to environmental exposures," said Tanya Froehlich, M.D., the study's lead author.

Dr. Froehlich's team monitored the bloodstream lead levels of 172 young children. At age five, they were tested for symptoms of AD/HD and for genetic variations, such as whether they carried a mutant form of the DRD4 dopamine receptor gene. This gene, which helps regulate levels of the neurotransmitter needed for attention and cognition, has been linked to ADHD.

As expected, the children with high lead levels showed impairment in planning, memory span, and "attentional flexibility" — the ability to revise a plan of action in the face of obstacles or new information. The children who carried the AD/HD-linked gene showed more impairment than children who did not carry the gene. Gender also played a role—exposure to lead affected boys more than girls.

"This study offers a model for examining how genes and environmental toxins interact to shape ADHD," Dr. Froehlich said, adding that such studies may help us understand "why certain groups may be more prone to ADD than others."

The study was presented last May at the annual meeting of the Pediatric Academic Societies in San Francisco.